This invention relates to semi-conductor switching circuits; more particularly, it relates to overload protection especially adapted for field effect transistor switching circuits such as those used as flashers for warning lamps on school busses.
It is common practice to provide school busses with warning lamp flashers such as amber caution lamps and red stop lamps. In such systems, the lamps which are flashed are typically incandescent lamps and, to produce the flashing effect, each lamp is turned on and off repetitively with a typical cycle of 400 milliseconds (ms) on-time and 400 ms off-time. Typically, a pair of amber lamps and a pair of red lamps are located on the front of the bus with one lamp of each color on the left side and one lamp of each color on the right side. A similar set of lamps is located at the rear of the bus.
It is well known that incandescent lamps exhibit a relatively low initial value of electrical resistance upon turn-on of the lamp and the resistance increases as the lamp filament temperature increases thus resulting in a high level inrush current which, after a short interval, subsides to a steady state value.
It is well known that power field effect transistors (FETs) are operable as power switching devices with very high power efficiency. However, the use of a FET for switching an incandescent lamp repetitively on and off requires special overcurrent protection for the FET due to the inrush current effect, short circuit or other transients. Operation of a FET at excessive or overcurrent values results in damage or destruction of the FET. Also, the use of a FET in a school bus flasher circuit module imposes a problem of damage or destruction of the FET resulting from a floating ground condition which may occur in the installation of the flasher module in the vehicle.
There is a need for a school bus flasher module which utilizes FETs for switching the lamp current and which provides protection of the FETs against operation at damaging current levels. Such a device must be of small size, reliable in operation and economical to manufacture.
A general object of this invention is to provide an improved flasher module, especially adapted for school busses, with FET switching devices which are protected against damaging current levels. Further, it is an object of the invention to provide a FET switching circuit useful in other applications with protection against damaging overcurrents.
In accordance with this invention, a school bus flasher circuit is provided in which a lamp load current is switched by a FET. A current sensing means generates a first signal voltage corresponding to actual lamp current and a load current limiting circuit generates a second signal voltage corresponding to a preset limit value of lamp current. A micro controller is responsive to the signal voltages and operates to disconnect the switching voltage from the gate of the FET when the first signal voltage exceeds the second signal voltage. The micro controller operates to change the reference voltage between a higher level during the lamp inrush current interval to a lower level during the steady state lamp current interval.